Light emitting diodes (LEDs) have been available since the early 1960's in various forms, and are now widely applied in a variety of signs and message boards. The relatively high efficacy of LEDs (in lumens per Watt) is the primary reason for their popularity. Tremendous power savings are possible when LED signals are used to replace traditional incandescent signals of similar luminous output. One aspect of LED technology that is not satisfactorily resolved is the application of LEDs in high temperature environments. LED lamps exhibit a substantial light output sensitivity to temperature, and in fact are permanently degraded by excessive temperature. Recent developments in LED technology have extended the maximum recommended operating temperature to 85.degree. C. These devices, which incorporate the element Indium in their chemistries exhibit typical (half brightness) lives on the order of 100,000 hours at 25.degree. C. However, degradation above 90.degree. C. is very rapid as the LEDs degrade exponentially with increases in temperature. The well known Arrhenius function approximately models this behavior, and predicts elevated temperature lifetimes of less than one year at temperatures approaching 100.degree. C. While such high temperatures might seem unusual for an LED operating environment, they are actually quite common. For example, traffic signal housings exposed to full summer sun can reach interior temperatures of 80.degree. C. without any lamp generated heat load. A thermal rise of only 20.degree. C., due to LED operation will stress the LEDs well beyond their sanctioned operating range.
Permanent thermal degradation of LEDs also occurs during array fabrication, when the LEDs are soldered to the supporting and/or interconnecting circuit board. Typical soldering temperatures (250.degree. C.) can significantly degrade the LED array before it is even put into service. LED manufacturers recommend the use of lead lengths of sufficient length to prevent excessive heat transmission from the soldering operation into the LED die. Of course, the added lead length acts detrimentally during LED operation, as the longer leads increase the thermal resistance and adversely affects the rejection of self generated heat. Surface mounted LEDs are even more difficult to solder without damage, as their leads are more closely thermally coupled to the LED die than in other package styles. The obvious dilemma is the need for good thermal coupling from the LED during operation to aid in heat extraction, while there is a need to limit heat transfer to the LED during soldering operations.
The need for thermally conductive substrates in a variety of high power density electronic products has led to the development of a number of unique substrate materials. These products generally perform a mechanical component support function, also provide for electrical interconnection to and between components, and optimally allow for the extraction and dissipation of component generated heat. Some of the more successful approaches include ceramic, non conductive cermet or even coated metallic substrates which are then laminated with copper, and are processed like conventional printed circuit boards. Thermally conductive ceramics are very costly compared to metal, and are reserved for very high temperature applications. The most common insulated metal substrates employ an aluminum or copper base, and a thin (20 micron) polyamide or resinous insulating coating that bonds the nominally 1 mil copper laminate to the substrate material. The effective thermal conductance of the dielectric insulator is relatively high because it is very thin. Of course the electrical insulating quality of the dielectric coating is important as it determines the maximum operating voltage of the circuit board `sandwich`. Puncture voltage and dielectric dissipation of the insulator coating is obviously a function of film thickness, and integrity. The difficulty in fabricating these composite, laminate, circuit boards, using dielectric films makes these components expensive when compared to conventional circuit boards. For critical, high performance uses this significant added cost is justified, but for uses that demand large substrates, the economics clearly do not favor these exotic composite, thermally conductive, laminate substrate materials. Also, the prior art attaches the LEDs to a free standing film or insulating substrate that is then laminated to the support structure or enclosure. Such lamination is not cost efficient, nor does lamination impart good thermal conductivity because of the inherent difficulty in achieving intimate contact between the thin film and the underlying substrate.
There are commercial products that have combined the use of films or sheets laminated to metallic substrates. One of the materials is "Koolbase" produced by Densitronics Corp., where a thin (1 oz.) copper foil is laminated with a 20 to 40 micron polyimide insulator to an aluminum base, by using heat and pressure. The resulting composite circuit board can be processed conventionally, and has excellent thermal properties; unfortunately, the process is very costly and cannot be applied to large circuit boards. A line of compact LED arrays manufactured by Mitsubishi Cable Col, called "Albaleds" use this composite substrate material. Another source for composite metallic substrates is the Berquist Corporation, which produces a rolled copper foil composite that is laminated to an aluminum substrate with an intervening proprietary insulation layer. This material is also rather costly and is processed like conventional printed circuit board material. That is, the necessary copper traces are formed by etching away the undesired copper by the traditional subtractive processes. The hybrid "chip on board", Mitsubishi LED array, is fabricated by traditional wire bonding techniques that are common to the semiconductor industry. The precision and tolerances required for hybrid fabrication limit the size of such LED arrays to about 100 cm.sup.2. Larger assemblies would have to be built up from a series of smaller hybrid arrays, further increasing manufacturing cost.
Various patents address the problem of heat dissipation in LED lamps but, in one way or another, each combination inherently includes an element with a high thermal resistance that impedes the dissipation of heat from the LEDs. The U.S. Pat. No. 4, 729,076 to Masami et al discloses an LED lamp assembly wherein a heat absorber, in the form of an electrically insulating sheet, is disposed between the circuit board holding the LEDs and the heat sink. Clearly, much thermal efficiency is lost in the use of such a high thermal resistance heat absorber. The U.S. Pat. No. 4,774,434 to Bennion discloses an LED disposed on a cloth shirt. A conductive epoxy material attaches the LED to a copper trace on a film substrate which is, in turn, adhesively secured to the cloth shirt. Obviously, the impediment to heat dissipation is the shirt since the shirt is not thermally conductive, i.e., cannot act as a heat sink. Accordingly, the use of a thermally conductive adhesive serves no thermally useful purpose and is unimportant and incidental. The U.S. Pat. No. 5,038,255 to Nishihashi et al discloses the laminating of a polyamid film substrate along with circuit patterns during the molding of an aluminum oxide filled resin support housing; alternatively, the circuit patterns are printed on the film. However, the use of a resinous material as the heat sink implies a thermal conductivity one hundred times lower than a metal substrate and therefore acts as an impediment bottleneck to the most efficient dissipation of heat. The conventional attachment of LED leads by soldering, as disclosed in U.S. Pat. No. 5,528,474 to Roney et al, is difficult to do without damaging the LEDs due to the high heat generated during the soldering operation and, even then, the long leads of the LEDs impede heat transfer during operation.